Ink jet printing is a well known poocess for non-contact printing of substrates such as paper, plastic films, metal foils and the like. In the operation of a typical ink jet printing apparatus a stream of an ink composition is forced under pressure through a very small orifice by a drive means such as a driven piezoelectric crystal. After the stream of the ink composition is forced from the orifice the stream separates into minute uniform size droplets. The droplets of the ink composition then receive an electrical charge. The charged droplets are deflected causing selected droplets to impinge upon the substrate to be printed in a predetermined pattern to form a dot matrix image of the desired indicia on the substrate.
Various types of ink jet printing apparatus are known in the art. One of the more common types of ink jet printing apparatus provides a constant discharge of droplets of the ink composition in substantial excess of the amount actually required for printing with the excess droplets being recovered and recycled. Another well known type of ink jet printing apparatus is referred to as a drop on demand printer. This type of apparatus, as the name indicates, discharges a stream of droplets of the ink composition only when required for printing. There are other types of ink jet printers known in the art and all typically operate on the same general principles as those described above.
The various types of apparatus used for ink jet printing are relatively well developed from a mechanical standpoint. However, considerable problems are still encountered with ink jet printers. Many of the problems are related to the ink compositions heretofore suggested for use in ink jet printing. Ink compositions which are used in ink jet printing should have certain properties. Initially in dispensing of the ink compositions during printing it is necessary that the ink compositions display a consistant breakup length, drop viscosity, and drop charge under the specific set of conditions employed during the ink jet printing process. To meet this requirement, the ink jet compostion must have stable viscosity and resistive properties and should not dry out upon aging. One of the major problems encountered with conventional prior art ink compositions is that they contain substantial amounts of either water or organic solvents which upon standing evaporate ink compositions to dry out and cake. This causes blocking the orifices and considerable other problems in operation of ink jet printers. A further problem is that the loss of the volatile solvents causes the inks to increase in viscosity which will cause substantial changes in the performance of the inks.
In addition to the unique requirements associated with ink jet printing it is also necessary that the ink compositions used for ink jet printing have the usual desired printing properties that can be obtained with inks used in conventional printing processes. These properties include, for example, that the ink composition have satisfactory adhesion to the various of different types of substrates on which it may be printed. In addition, the indicia which is printed must have a high degree of resolution in order to provide the sharp images required for most printing applications. The ink composition must not rub or smudge on the substrate and the printed indicia should likewise be permanent; that is, of archival quality.
A further problem in addition to those noted above associated with the presence of volatile solvents in the prior ink compositions used for ink jet printing is that the solvents cause the printed indicia to bleed into the substrates and result in poor resolution. To overcome this, the prior art resorted to the expedience of using coated papers which will not be wet out by the ink compositions. This is generally limited in practice, because of the additional cost involved and the requirement in many commercial printing process for "plain paper" printing. The use of large amounts of solvents in the ink composition likewise required that the printed substrates be subjected to drying to remove the solvents. In the case of organic solvents this has caused additional health and safety concerns.
Certain of the prior art ink compositions in addition to containing solvents also contain substantial amounts of wetting agents to allow the ink copposition to wet out and saturate the substrate to be printed. These particular ink compositions tend to excessively wet out the substrates with solvents and in turn cause bleeding with a resulting substantial reduction in the resolution of the printed indicia.
In typical conventional ink compositions for most other printing applications that is aside from ink jet printing, pigments are used as the colorants. Pigments are preferred to dyes because of the cost, consistent color quality and more importantly becuase of their outstanding permanency. However, because of the problems encountered with the prior art ink compositions used in ink jet printing and particularly because of the drying out of the solvents from prior art ink compositions it has not been possible to successfully use pigments in ink compositions for ink jet printing.
What would be highly desirable would be an ink composition for ink jet printing which would be stable under ink jet printing conditions and which provides excellent print qualities such as high resolution pattern with a high contrast, excellent adhesion to the substrate and also excellent archival properties.